In synthetic organic chemistry, hydrogenation reactions are generally carried out in two types of processes:
(A) IN THE VAPOR PHASE BY THE PASSAGE OF A STREAM OF GASEOUS HYDROGEN AND OF THE COMPOUND TO BE HYDROGENATED ONTO A METALLIC CATALYST; AND
(B) IN THE LIQUID PHASE BY THE ACTION OF A HYDROGENATING AGENT SUCH AS HYDROGEN, SODIUM BOROHYDRIDE, BORON HYDRIDE, MAGNESIUM HYDRIDE OR LITHIUM-ALUMINUM HYDRIDE, OR THROUGH THE INTERMEDIARY OF HOMOGENEOUS HYDROGENATION CATALYSTS.
In the first case, the parameters which have an effect on the selectivity and the kinetics (reaction rate) of the vapor phase reaction are: temperature, total pressure and the partial pressure of the reactants, and the physical characteristics of the catalytic surface.
In practice, the purity of the hydrogen plays an important role since it can determine the useful life and the performance of the catalyst.
In the second phase (liquid phase hydrogenation), there are two possibilities:
(A) If the hydrogenating agent is stable in the solvent used, its use is able to preclude reactions whose free enthalpy is greater than the free enthalpy of formation of the hydrogenating agent.
(B) If the hydrogenating agent is decomposed by the solvent, the organic compound is reduced by the hydrogen freed in the reaction and the reaction becomes uncontrollable.
Furthermore, the hydrogenating agents generally used are not regeneratable but produce byproducts necessitiating operations for their removal from the system.
Thus it has been of particular interest to make use of hydrogenating agents which are regeneratable and thus do not give rise to decomposition byproducts.
There has been proposed an electrohydrogenation process which is effected with a cathode releasing hydrogen and placed in contact with the compound to be hydrogenated. However, the formation of gaseous hydrogen at the cathode readily blocks a large number of the active sites so that chemisorbtion greatly limits the overall hydrogenation kinetics.
Because of the parasitic release of hydrogen which accompanies hydrogenation, there is a significant reduction in the degree of hydrogenation and it becomes necessary to remove the excess hydrogen from the reaction medium.
In U.S. Pat. No. 3,382,106, there is described a process for producing a catalytic body which is utilizable to catalyze reactions based upon the transfer of hydrogen, especially hydrogenation reactions and dehydrogenation reactions in a galvanic cell.
This process resides in imparting a catalytic activity to a body of a metal or an alloy having a metallic crystal lattice which absorbs the hydrogen. To this end, the body is placed in contact with an alkaline solution comprising a complex reducing hydride releasing atomic hydrogen which is absorbed in the metallic crystalline lattice and is capable of forming a hydride therewith.
According to this patent, the same process can be used to reactivate the catalytic body when its catalytic activity diminishes.
Thus, while one is able, using this system, to provide a satisfactory catalyst for the reaction desired and which is reproducible to a degree, it is generally necessary to replace the catalyst after it has lost a significant part of its requisite catalytic property. The replacement is more or less frequent depending upon the nature of the catalyst and the other members of the reaction system and thus involves an interruption of the catalytic reaction at significant cost.
It has also been proposed to use a hydride of an alloy of titanium and nickel as a catalyst for hydrogenation reactions or dehydrogenation reactions, the catalyst being effective in a gaseous phase or a liquid phase. Such catalyst is able to be reused and to be regenerated chemically or electrochemically to restore the requisite catalyst properties. However, the more or less frequent replacement of the catalyst and its regeneration also is inconvenient even if only a portion of the catalyst is replaced at each time. As a result a significant quantity of the catalyst must be immobilized which can have an adverse effect on the reaction and the cost is again considerable.